The present invention relates to a transformer assembling method, a transformer, and a transformer-mounted substrate, or to an art most preferably applicable to, for example, a transformer for switching power supply units using a ferrite core. More particularly, this invention is concerned with a power supply unit having a components-mounted substrate or a power supply unit included in a recording apparatus, for example, an AC adaptor used as a power supply for electronic equipment or a power supply unit connected to the AC adaptor.
In general, a switching power supply unit can be designed to be more compact and lightweight than a dropper power supply unit, and has the advantage of exhibiting high conversion efficiency. The switching power supply unit is therefore widely adopted for various usages ranging from the usage as a built-in power supply to be incorporated in a compact equipment to the usage as an AC adaptor. Since the switching power supply unit has thus prevailed, various kinds of electronic equipment that are compact and lightweight can be readily carried about with females or children. The contribution of the switching power supply unit to industries has been highly appreciated.
Referring to the drawings, a transformer to be mounted on a substrate for this kind of switching power supply unit will be described briefly. In the front view of FIG. 46, a coil 103 is formed on a bobbin 102, and a core 101 is inserted into the bore of the bobbin 102a. Thereafter, as shown in FIG. 47, an adhesive is applied to illustrated regions or the transformer is impregnated with a varnish as shown in FIG. 48. Thus, a finished good is completed.
Using the above components, the transformer 100 is designed to be relatively flat, and mounted on a substrate. Thus, the transformer contributes to a compact and lightweight design of a power supply.
However, when the transformer 100 designed to be compact and lightweight and mounted on a substrate is incorporated in a power supply unit, there arises a problem that it may incur an excessive impact load because of improper handling during transportation.
Moreover, when the transformer 100 designed to be compact and lightweight and mounted on a substrate is incorporated in a power supply to be used as an AC adaptor placed on a desktop, since a power cord may be hooked to cause such an event that the power supply falls off the desktop, unprecedented measures must be taken in order to enable the power supply to resist an excessive impact load.
As already known, when an impact load works in this way, the excessive impact load is concentrated on the transformer whose weight is larger than the other various electronic components of the power supply.
The broken state of a transformer is serious when an external force works, as shown in FIG. 43, on a components-mounted substrate 20 in a direction of an arrow F. In other words, when an external force works, a moment oriented in a direction of an arrow M works on a transformer 100. A tensile force acts abruptly on one of thin parts 102a of the bobbin 102 firmly soldered to the pattern-printed area of the substrate 20 via lead pins 12 connected to the secondary winding of the transformer 100. As a result, as already known, a breakdown phenomenon or a phenomenon that a transformer is ruptured at the thin part 102a thereof takes place.
A countermeasure against the rupture is such that the thin parts 102a united with both ends of the coil 103 formed on the bobbin are, as shown in FIG. 44A that is the front view of the bobbin 102 and FIG. 44B that is the right-hand side view of the bobbin, chamfered for the purpose of reinforcement. The anti-breakdown strength of the bobbin 102 is thus improved.
Another fixing method proposed is, as disclosed in Japanese Unexamined Patent Publication Nos. 6-96965 and 6-163278, such that a transformer itself is stowed in a housing for holding the whose transformer and gaps are filled with a resin.
Moreover, according to Japanese Unexamined Patent Publication No. 9-214156, ribs are interposed between radiating plates that are components to be mounted on a substrate. Even when the substrate is dropped, the ribs prevent the substrate from deforming to an extent exceeding a limit. The anti-impact resistivity of the substrate is thus improved.
Furthermore, in many cases, a containing means included in a power supply unit is composed of an upper case and lower case which are separated from each other up and down with a substrate between them. Supporting members are extended from the upper case and lower case in order to immobilize the substrate horizontally and vertically.
The sectional view of FIG. 45 shows an example of a known holding structure for a substrate. There are shown upper and lower cases 213 and 214 for an AC adaptor, a substrate 220 to be held by the cases, screws 221 for tightening the upper case 213 and lower case 214, a transformer 100, radiating plates 222 and 223, and an aluminum electrolytic capacitor 225. On each radiating plate, heat-dissipation components, for example, a power transistor, FET, and diode are fixed using screws.
Ribs 213b that are supporting members successively extending from the upper case 213 are abutting on the substrate 220 and thus locking it. Ribs 214b are successively formed on the lower case 214, and abutting on and holding the bottom of the substrate 220. This helps the substrate 220 resist an impact load including a drop, and prevents deformation-based breakdown of the substrate 20.
However, according to the known structure shown in FIG. 45, many members such as the ribs must be formed inside the cases in order to prevent deformation of the substrate 20 as greatly as possible.
Furthermore, in many cases, a containing means of a power supply unit is composed of an upper case and lower case that are separated from each other up and down with a substrate between them. Supporting members are extended from the upper case and lower case in order to immobilize the substrate horizontally and vertically.
Referring to the drawings, a transformer to be mounted on a substrate for a switching power supply will be reiterated. FIG. 46 is a sectional view showing a major portion of a transformer 100 that has been generally adopted in the past. The transformer and a core are impregnated with a varnish and thus secured. In this drawing, a coil 103 is formed on a bobbin 102, and a core 101 is inserted into the bore of the bobbin 102. Thereafter, adhesives 105 and 106 are applied to illustrated regions. This results in a finished good. In FIG. 48, the whole outer circumference of the transformer is coated with a varnish 108, whereby a finished good is completed.
In the above structures, the transformer 100 is placed to be relatively flat and then mounted on a substrate. The transformer contributes to a compact and lightweight design of a power supply.
Next, a transformer to be mounted on a substrate for a switching power supply unit will be described briefly. FIGS. 47 and 48 are oblique views of the appearance of a transformer 100 that has generally been adopted in the past. FIG. 47 shows a transformer of a type having a core secured using adhesives, and FIG. 48 shows a transformer of a type having a core secured using a varnish with which the transformer is impregnated.
In FIG. 47, a coil 103 is formed on a bobbin 102, and a core 101 is inserted into the bore of the bobbin 102. Thereafter, adhesives 105 and 106 are applied to illustrated regions in order to secure the core, whereby a finished good is completed. In FIG. 48, the whole outer circumference of the transformer is coated with a varnish 108, whereby a finished good is completed.
Whichever of the structures is adopted, the transformer 100 is designed to be relatively flat and then mounted on a substrate. The structure constitutes to a compact and lightweight design of a power supply.
However, there is a problem that after the transformer 100 shown in FIG. 46, 47, or 48 is designed to be compact and lightweight and then mounted on a substrate, when the substrate is incorporated in a power supply unit, the substrate may incur an excessive impact load because of improper handling during transportation.
When a power supply including the transformer 100 designed to be compact and lightweight and mounted on a substrate is used as an AC adaptor placed on a desktop, a power cord may be hooked to cause such an event that the power supply falls off the desktop. This makes it necessary to take unprecedented measures.
As already known, when an impact load thus works, an excessive impact load is concentrated on the transformer, which is heavier than the other various electronic components of the power supply unit.
The broken state of a transformer is serious when an external force works, as shown in FIG. 43, on a components-mounted substrate 20 in a direction of an arrow F. In other words, when an external force works, a moment oriented in a direction of an arrow M acts on the transformer 100. At this time, a tensile force is applied abruptly to one of thin parts 102a of the bobbin 102 fixed to a pattern-printed area of the substrate 20 by way of lead pins 11 and 12 of the transformer 100. As a result, a breakdown phenomenon or a phenomenon that a transformer is ruptured at the thin part 102a takes place.
A countermeasure against the rupture is such that the thin parts 102a united with both ends of the coil 103 formed on the bobbin are chamfered for the purpose of reinforcement. The anti-breakdown strength of the bobbin 102 is thus improved.
Another fixing method is, as disclosed in Japanese Unexamined Patent Publication No. 6-96965 or 6-163278, such that a transformer itself is stowed in a housing case for holding the whole transformer and gaps are filled with a resin.
However, the method for improving anti-breakdown strength by chamfering the thin parts 102a of the bobbin 102 has a problem that since the precision in outer dimensions of a ferrite core employed greatly differs from one to another, a gap may be created by the side of a ferrite core and the transformer may therefore fail to resist an excessive impact load such as a drop.
Moreover, supposing the whole bobbin 102 were made thicker in order to improve the strength of the bobbin 102 and chamfered sufficiently, an effective window of a large area cannot be defined for the transformer. This poses a problem that the performance of the ferrite core inserted into the bore of the bobbin cannot be exerted fully.
As disclosed in Japanese Unexamined Patent Publication No. 7-300561, a method using a resin, which enjoys high mechanical strength, as a material for a bobbin has been proposed. However, a material usable for the bobbin of a transformer must clear the UL standards and other safety standards stipulated by each nation. The material satisfying these standards costs considerably higher than any known material. From the viewpoint of the use practice in markets, there is a problem that the use of an unprecedented material is accompanied by a great risk.
Furthermore, as described in the Japanese Unexamined Patent Publication Nos. 6-96965 and 6-163278, the method of stowing the whole transformer in a housing case and sealing the housing case with a resin has drawbacks that extra cost is required and that the merit of a compact and lightweight design characterizing a switching power supply unit cannot be fully exerted. By the way, as disclosed in Japanese Unexamined Patent Publication No. 7-300561, a method using a resin, which enjoys high mechanical strength, as a material of a bobbin has been proposed. However, since a material to be made into a bobbin of a transformer must clear the UL standards and other safety standards stipulated by each nation. The material satisfying all the standards costs considerably higher than known materials. From the viewpoint of the use practice in markets, there is a problem that the use of an unprecedented material is accompanied by a great risk.
Furthermore, according to the known structure shown in FIG. 45, many members such as ribs must be formed within the cases in order to restrain the substrate 20 from deforming as greatly as possible.
(a) A change in thickness of an upper or lower case and a change in flow of a molding material gets larger to affect the quality of the cases.
(b) The contact state of ribs to a substrate becomes different from one substrate to another because of the uncertainty in precision of the heights of the ribs. Some of the ribs may not work as supporting members. Moreover, when some ribs are jutting out of other ribs and abutting on the substrate, a concentrated stress is applied to the substrate. The applied region of the substrate may crack.
(c) When the number of supporting members that are ribs increases, deformation of a substrate caused by an impact load will be avoided reliably. However, an impact load calculated by multiplying the weights of components mounted on the substrate by an acceleration does not vary at all. The load is therefore concentrated on soldered parts or fixtures for fixing mounted components to the substrate. This poses a problem that a land may be peeled off or a cohesive failure of solder may occur. This trend is outstanding in relation to a heavy transformer.